Men and women 20 to 75 years old with HCM may be eligible for this study. Participants will undergo a physical examination, blood tests, and other tests and procedures, described below, to assess heart function. When the tests are completed, patients will be randomly assigned to one of two treatment groups. One group will take a pirfenidone capsule and the other will take a placebo (a look-alike pill with no active ingredient) twice a day with meals for 6 months. For the pirfenidone group, the dose of drug will be increased gradually from 400 to 800 milligrams. At the end of 6 months, all patients will repeat the physical examination and heart tests that were done before starting medication. These include: